As worldwide electric power demands continue to increase significantly, utilities have struggled to meet these increasing demands both from a power generation standpoint as well as from a power delivery standpoint. Delivery of power to users via transmission and distribution networks remains a significant challenge to utilities due to the limited capacity of the existing installed transmission and distribution infrastructure, as well as the limited space available to add additional conventional transmission and distribution lines and cables. This is particularly pertinent in congested urban and metropolitan areas, where there is very limited existing space available to expand capacity.
Power cables using high temperature superconductor (HTS) wire are being developed to increase the power capacity in utility power transmission and distribution networks, while maintaining a relatively small footprint. For this disclosure, HTS is defined as a superconductor with a critical temperature at or above 30° Kelvin, which includes materials such as rare-earth- or yttrium-barium-copper-oxide; thallium-barium-calcium-copper-oxide; bismuth-strontium-calcium-copper-oxide; mercury-barium-calcium-copper-oxide; and magnesium diboride. Such HTS cables allow for increased amounts of power to be economically and reliably provided within congested areas of a utility power network, thus relieving congestion and allowing utilities to address their problems of transmission and distribution capacity.
An HTS power cable uses HTS wire as the primary conductor of the cable (i.e., instead of traditional copper conductors) for the transmission and distribution of electricity. The design of HTS cables results in significantly lower series impedance, when compared to conventional overhead lines and underground cables. Here the series impedance of a cable or line refers to the combination of resistive impedance of the conductors carrying the power, and the inductive impedance associated with the cable architecture or overhead line. For the same cross-sectional area of the cable, HTS wire enables a three to five times increase in current-carrying capacity when compared to conventional alternating current (AC) cables; and up to a ten times increase in current-carrying capacity when compared to conventional direct current (DC) cables.
In addition to capacity problems, another significant problem for utilities resulting from increasing power demand (and hence increased levels of power being generated and transferred through the transmission and distribution networks) are increased “fault currents” resulting from “faults”. Faults may result from network device failures, acts of nature (e.g. lightning), acts of man (e.g. an auto accident breaking a power pole), or any other network problem causing a short circuit to ground or from one phase of the utility network to another phase. In general, such a fault appears as an extremely large load materializing instantly on the utility network. In response to the appearance of this load, the network attempts to deliver a large amount of current to the load (i.e., the fault).
Detector circuits associated with circuit breakers monitor the network to detect the presence of a fault (or over-current) situation. Within a few milliseconds of detection, activation signals from the detector circuits may initiate the opening of circuit breakers to prevent destruction of various network components. Currently, the maximum capability of existing circuit breaker devices is 80,000 amps. Many sections of the utility network built over the previous century were built with network devices capable of withstanding only 40,000 to 63,000 amps of fault current. Unfortunately, with increased levels of power generation and transmission on utility networks, fault current levels are increasing to the point where they will exceed the capabilities of presently installed or state-of-the-art circuit breaker devices (i.e. be greater than 80,000 amps). Even at lower fault current levels, the costs of upgrading circuit breakers from one level to a higher one across an entire grid can be very high. Accordingly, utilities are looking for new solutions to deal with the increasing level of fault currents. One such solution in development is a device called an HTS fault current limiter (FCL).
An HTS FCL is a dedicated device interconnected to a utility network that reduces the amplitude of the fault currents to levels that conventional, readily available or already installed circuit breakers may handle. Unfortunately, such standalone HTS FCLs are currently quite large and expensive. Utilities may also use large inductors, but they may cause extra losses, voltage sag and grid stability problems. And, unfortunately, pyrotechnic current limiters (e.g., fuses) need replacement after every fault event. Further, while new power electronic FCLs are under development, they are also expected to be large and expensive.
To protect HTS cables against fault currents, a significant amount of copper is introduced in conjunction with the HTS wire, but this adds to the weight and size of the cable. Often, copper fills the central former in the core of the HTS cable around which the HTS wire is spirally wound, and this prevents the core from being used as a passage for the flow of liquid nitrogen.
It is desirable to improve the way HTS cables handle fault currents and to provide an improved alternative to the use of standalone FCLs or other fault current limiting devices.